Hans Adolf Krebs

Krebs was born in Hildesheim, Germany, to Georg Krebs, an ear, nose, and throat surgeon, and Alma Krebs (née Davidson). He descended from Jewish-Silesian ancestry and was the middle of three children, older sister Elisabeth and younger brother Wolfgang.

Krebs attended the famous old Gymnasium Andreanum in his home town. Near the end of World War I, in September 1918, six months short of completing his secondary school education, he was conscripted into the Imperial German Army. He was allowed to take an emergency examination for his high school diploma, which he passed with such a high score that he suspected the examiners of being "unduly lenient and sympathetic".[8] With the end of the war two months later, his conscription ended.

Krebs decided to follow his father's profession and entered the University of Göttingen in December 1918 to study medicine. In 1919 he transferred to the University of Freiburg. In 1923 he published his first scientific paper on a tissue staining technique. He did this work under the guidance of Wilhelm von Mollendorf starting it in 1920. He completed his medical course in December 1923. To obtain a Doctor of Medicine degree, and a medical license, he spent one year at the Third Medical Clinic in the University of Berlin. By then he had turned his professional goal from becoming a practising physician to becoming a medical researcher, particularly in biochemistry. In 1924 he studied at the Department of Chemistry at the Pathological Institute of the Charité Hospital, in Berlin, for training in chemistry and biochemistry. He earned his MD degree in 1925 from the University of Hamburg.[9][10]

In 1926 Krebs joined Otto Heinrich Warburg as a research assistant at the Kaiser Wilhelm Institute for Biology in Dahlem, Berlin. He was paid a modest 4800 marks per year. After four years in 1930, with 16 publications to his credit, his mentor Warburg urged him to move on and he took up the position of Assistant in the Department of Medicine at the Municipal Hospital in Altona (now part of Hamburg). The next year he moved to the Medical Clinic of the University of Freiburg. At Freiburg he was in charge of about 40 patients, and was at liberty to do his own research. Before a year was over at Freiburg, he, with research student Kurt Henseleit, published their discovery of the ornithine cycle of urea synthesis, which is the metabolic pathway for urea formation. It is now known as the urea cycle, and is sometimes also referred to as the Krebs–Henseleit cycle. Together they also developed a complex aqueous solution (a buffer), or perfusion ex vivo, for studying blood flow in arteries, which is now called the Krebs–Henseleit buffer.)[11][12] In 1932 he published the basic chemical reactions of urea cycle, which established his scientific reputation.

Krebs's life as a respected German scientist came to an abrupt halt in 1933 because of his Jewish ancestry. With the rise of Hitler's Nazi Party to power, Germany decreed the Law for the Restoration of the Professional Civil Service, which decreed the removal of all non-Germans, and anti-Nazis, from professional occupations. Krebs received his official dismissal from his job in April 1933, and his service was terminated on 1 July 1933. An admirer, Sir Frederick Gowland Hopkins at the University of Cambridge, immediately came to his rescue, and persuaded the university to recruit Krebs to work with him in the Department of Biochemistry.[13] By July 1933 he was settled in Cambridge with financial support from the Rockefeller Foundation. Although Germany restricted him to bringing only his personal belongings, he was fortunate that the government agents allowed him to take his equipment and research samples to England. They proved to be pivotal to his later discoveries, especially the manometer developed by Warburg specifically for the measurement of oxygen consumption in thin slices of tissues; it was the basis for his research.[14] He was appointed as Demonstrator in biochemistry in 1934 and in 1935 the University of Sheffield offered him a post of Lecturer in Pharmacology, with a more spacious laboratory and double the salary. He worked there for 19 years. University of Sheffield opened a Department of Biochemistry, now Department of Molecular Biology and Biotechnology, in 1938 and Krebs became its first Head, and eventually a Professor in 1945. Krebs took over the running of the Sorby Research Institute in 1943. In 1944, the British Medical Research Council established the MRC Unit for Cell Metabolism Research at Sheffield, and Krebs was appointed the Director. With this his laboratory became so large that the locals jokingly nicknamed it "Krebs's Empire". He moved with his MRC unit to the University of Oxford in 1954 as Whitley Professor of Biochemistry, the post he held till his retirement in 1967. The editorial board of Biochemical Journal extended their good wishes on his retirement, but in return he promised to keep them busy, by producing scientific papers. He continued his research, and took his MRC unit to the Nuffield Department of Clinical Medicine at the Radcliffe Infirmary, Oxford. From there he published over 100 research papers.[9][10][15][16]

Krebs with wife in Stockholm in 1953

Krebs met Margaret Cicely Fieldhouse (30 October 1913 – May 1993)[17] when he moved to Sheffield in 1935. They married on 22 March 1938. Krebs later described his life in Sheffield as "19 happy years".[9] They had two sons, Paul (born 1939) and John (born 1945), and a daughter, Helen (born 1942).[18] John (Sir John Krebs, and later Baron Krebs) became a renowned ornithologist, Professor at the University of Oxford, Principal of Jesus College, Oxford, and Member of the British House of Lords.[19]

After a brief illness, Krebs died on 22 November 1981 in Oxford.[3][20]

In 1932 Krebs worked out the outlines of the urea cycle with a medical student Kurt Henseleit at the university of Freiburg. While working at the Medical Clinic of the University of Freiburg, Krebs met Kurt Henseleit, with whom he investigated the chemical process of urea formation. In 1904, two Germans A. Kossel and H. D. Dakin had shown that arginine could be hydrolysed by the enzyme arginase to form ornithine and urea in inorganic reaction.[21] Based on this reaction, Krebs and Henseleit postulated that in living cells, similar reaction could occur, and that ornithine and citrulline could be the intermediate reactions.[22][23] Krebs started working on the possible method for the synthesis of arginine. Using his Warburg manometer, he mixed a slice of liver with purified ornithine and citrulline. He found that citrulline acted as a catalyst in the metabolic reactions of urea from ammonia and carbon dioxide. He and Henseleit published their discovery in 1932. Thus the urea cycle (or "ornithine cycle") was established, and it was the first metabolic cycle to be discovered.[14][24]

At the University of Sheffield, Krebs and William Johnson investigated cellular respiration by which oxygen was consumed to produce energy from the breakdown of glucose. Krebs had earlier suggested to Warburg while they worked together in Germany that by using a manometer it could be possible to detect the oxygen consumption and identify the chemical reaction in glucose metabolism. Warburg had flatly rejected the idea. In Sheffield Krebs vigorously worked to identify a possible chemical reaction and came up with numerous hypothetical pathways. Using the manometer he tested those hypotheses one by one. One hypothesis involving succinate, fumarate, and malate proved to be useful because all these molecules increased oxygen consumption in the pigeon breast muscle. In 1937 German biochemists Franz Koop and Carl Martinus had demonstrated a series of reactions using citrate that produced oxaloacetate. Krebs realised that these molecules could be the missing intermediates for such reaction. After four months of experimental works to fill in the gaps, Krebs and Johnson succeeded in establishing the sequence of the chemical cycle, which they called the "citric acid cycle".[25][26] It is also known as the "Krebs cycle" or "tricarboxylic acid (TCA) cycle".

Krebs sent a short manuscript account of the discovery to Nature on 10 June 1937. On 14 June he received a rejection letter from the editor, saying that the journal had "already sufficient letters to fill correspondence columns for seven or eight weeks", and encouraging Krebs to "submit it for early publication to another periodical."[27] Krebs immediately prepared a longer version titled "The Role of Citric Acid in Intermediate Metabolism in Animal Tissues", which he sent to the Dutch journal Enzymologia after two weeks and was published in two months.[3][28] It was followed by a series of papers in different journals.[29][30][31]

Krebs continued to add more details to his citric acid cycle. The discovery of acetyl-CoA in 1947 by Fritz Albert Lipmann was another major contribution.[4][32] However, this new discovery posed a problem in his classic reaction. In 1957 he, with Hans Kornberg, found that there were additional crucial enzymes. One was malate synthase, which condenses acetate with glyoxylate to form malate, and the other was isocitrate lyase, which provides glyoxylate for the reaction by cleaving it from isocitrate.[33] These two reactions did not follow the normal citric acid cycle, and hence the pathway was named the glyoxylate bypass of the citric acid cycle, but is now known as the glyoxylate cycle.[14][34]